The present invention relates to a fluidized bed reactor and method of operating the fluidized bed reactor. More specifically the invention relates to controlled operation of a circulating fluidized bed reactor that has a number of advantages compared to prior art constructions and processes. The invention is greatly simplified compared to the prior art yet allows precise control of the temperature of the reactor in an efficient manner, increases heat transfer capacity at different loads, and is otherwise advantageous.
Circulating fluidized beds are well known, such as shown in U.S. Pat. No. 4,111,158. In circulating fluidized bed reactors fuel is reacted in a fluidized bed of solid (inert, and/or active--such as limestone) particles. The gas velocities and rate of gas feeds are controlled to such a manner that substantial portion of the solid particles are entrained with the gas flowing in the fluidized bed reactor from a lower part to an upper part. It is characteristic for operation of circulating fluidized bed that entrainment of solid material is so extensive that if that material (or an equal amount of preheated material) is not recirculated back to the reactor the operation of the circulating fluidized bed is adversely affected.
It is suggested in U.S. Pat. No. 4,111,158 that the temperature and operation may be controlled by withdrawing solids from the circulation system, [consisting of a fluidized bed reactor, solids/gas separator and recycling conduit], cooling the withdrawn solids by fluidized bed heat exchanger, and then recycling cooled solids back to the fluidized bed reactor. The solids are withdrawn from near the bottom of the fluidized bed reactor via a conduit, and pass to an external remote fluidized bed cooler, and after cooling the solids part of them are turned to the fluidized bed reactor. Such an arrangement requires a separate system to have the solids transported between the fluidized bed cooler and the fluidized bed reactor. Moreover, its controlling capacity is poor, e.g. due to long conduits for transporting the solids, which long conduits also stiffer large heat losses. Such a system is also very complicated and expensive to manufacture and operate.
It has also been suggested to provide the solids cooled in a fluidized bed adjacent the main reactor, e.g. in an article in VORTEX.TM. FLUIDIZED BED TECHNOLOGY, ASME 1993, Fluidized Bed Combustion--Volume 1, pages 197-205. With such an arrangement it may be possible to diminish thermal losses and control delay of long connection conduits, but it still requires some device to return the solids to the main reactor, which in this case is a separate lift channel, which also requires additional power to operate. The solids are taken from the bottom of the reactor and the return of solids back to the reactor is realized by a separate lifting chamber, used to prevent the mixing of fluidization gas of the cooler and the conveying gas of the lifting chamber with each other. Also, control of such a system is difficult; there must always be an adequate volume of solids introduced into the lifting chamber or otherwise transportation is not successful.
In U.S. Pat. Nos. 4,893,426 and 4,823,740 there are disclosed different approaches to operation of a bubbling fluidized bed reactor. Bubbling fluidized bed reactors are operated at low velocities, so that a distinct upper surface of the bed is formed, contrary to circulating fluidized beds. In U.S. Pat. No. 3,893,426 there is shown a heat exchanger utilizing adjacent fluidized beds. Both beds have their fluidization gas distributing gird at the same horizontal level. In U.S. Pat. No. 4,823,740 there is shown a bubbling fluidized bed reactor wherein the lower part of the reactor is provided with thermal energy recovery chambers. These chambers are positioned substantially at same level with the bubbling bed in order to be capable of receiving solid material which enters above the top surface of the bubbling bed near the partition wall separating the bubbling bed and the recovery chamber. Solids are returned from the recovery chamber to the bubbling bed at a level above the fluidization grid of the bubbling bed.
U.S. Pat. No. 5,060,599 shows a circulating fluidized bed reactor having pockets formed in a sidewall of the reactor to receive material flowing downwardly along the wall. Each pocket is provided with an upward opening at a location where the density of the fluidized bed is considerably lower that adjacent the reactor bottom. Control of the material flow is accomplished by allowing the material to outflow over the edge of the pocket, or material is discharged via a duct or opening in the bottom of the pocket. The pocket is formed by providing a partition wall in the reaction chamber. To have sufficient volume for the pocket and heat transfers therein the partition wall must be relatively high up on the wall. This kind of heavy wall structure is very expensive and difficult to construct, it causes stresses to other structures at its connection locations, and causes undesired vibration of structures. If the height of the partition wall is increased the operation of the pocket will be restricted merely to high load operation, since at low loads not enough solid material will be falling into the pocket.
In U.S. Pat. No. 4,363,292 there is shown a system to provide heat transfer sections on the bottom grid of a fluidized bed reactor. In this system there are also partition walls provided above the grid to divide the bottom section of the reactor into several sections. This arrangement is also limited by its inability to provide a sufficient amount of heat transfer surface in the heat transfer section, particularly for low load conditions. In this and other known systems and methods for operating fluidized be reactors there are shortcomings which the present invention solves.
According to the present invention a system and method are provided that allow the temperature of the fluidized bed reactor to be controlled efficiently, allowing adequate heat transfer surface area for cooling of solid material. According to the invention it is possible to increase heat transfer capacity of the fluidized bed reactor at different loads and to provide effective and economical treatment of the solid material in the fluidized bed reactor. The heat transfer capacity of the reactor system cooler increases compared to the prior art, allowing effective operation at different loads. Yet these results are accomplished according to the present invention in a simple manner.
The most basic concept behind the present invention is the utilization of two different fluidized bed technologies, and the mounting of the two different beds in juxtaposition to each other so as to allow a mutual exchange of particles between the beds without requiring pumps, blowers, or other mechanical or pneumatic equipment to directly accomplish the exchange of particles.
The invention utilizes a circulating (fast) fluidized bed and a bubbling (slow) fluidized bed. The beds are mounted adjacent each other with first and second interconnections between them, typically with the fluidizing gas introducing grid of the bubbling bed below that of the circulating bed. Because the bubbling bed has substantially constant density throughout, with a clear demarcation line at the top thereof, the first interconnection is provided above the top of the bubbling bed so that the pressure and density conditions between the beds result in a flow of particles from the circulating bed to the bubbling bed through the first interconnection. However since the average density in the bubbling bed is higher than the density in the circulating bed, the pressure and density conditions cause the particles after treatment in the bubbling bed (e.g. after cooling) to return to the circulating bed through the second interconnection.
That is, it has been surprisingly found that it is possible to efficiently utilize different pressure conditions existing in a fluidized bed reactor system in order to transport material between two fluidized beds of solids. By appropriately arranging the chambers and openings through which they communicate with each other it is possible to maintain and control the operation of fluidized bed reactor so that efficient cooling of solid material is established in a safe and reliable manner at all operating loads. The present invention operatively connects a circulating fluidized bed and a slow fluidized bed to achieve these results.
In a circulating fluidized bed of solids, fluidization gas is introduced through a grid at the bottom of reactor chamber at such a rate that a considerable volume of solids is entrained with gas moving frown a lower section of the reaction chamber to an upper section thereof. Also, it is characteristic of circulating beds that the mean particle density gradually decreases toward the upper section of the reaction chamber beginning from an initial density of circulating fluidized bed at its bottom section, and there is not any distinct upper surface on the bed, rather the gas/solids suspension dilutes gradually upwardly. On the other hand, in a slow or bubbling fluidized bed there is a distinct upper surface below which the particle density is substantially constant and above which only insignificant amount of solids are present; i.e., the solid density is substantially zero above the upper surface. This is due to the relatively low rate of introduced fluidization gas.
According to the invention, the density of the slow fluidized bed is typically provided as greater than the initial density of circulating fluidized bed at its bottom section. These fluidized beds establish pressures which may be described by .DELTA.p.sub.1 =.rho..sub.c g .DELTA.h or by pressure gradient .DELTA.p.sub.1 /.DELTA.h for a circulating fluidized bed, and .DELTA.p.sub.2 =.rho..sub.s g .DELTA.h or by pressure gradient .DELTA.p.sub.2 /.DELTA.h for a slow or bubbling fluidized bed. In the slow fluidized bed the density will naturally dramatically drop at the height of the upper surface of the fluidized bed and thus the pressure .DELTA.p.sub.2 will not increase above the upper surface of the slow fluidized bed--this height is designated as h.sub.o. On the other hand, since the mean particle density in the circulating fluidized bed gradually decreases towards the upper section of the reaction chamber, there is no such dramatic change in a circulating fluidized bed. These lead to the fact that at a vertical location below the upper surface of the slow fluidized bed, at height .DELTA.h.sub. 1 being equal to or less than h.sub.o, the pressure of the slow fluidized bed is greater than the pressure of the circulating fluidized bed, i.e. .DELTA.p.sub.2 &gt;.DELTA.p.sub.1. And respectively, at a vertical location above the upper surface of the slow fluidized bed, at height .DELTA.h.sub.u which is greater than h.sub.o, the pressure of the circulating fluidized bed is greater than the pressure of the slow fluidized bed, i.e. .DELTA.p.sub.1 &gt;.DELTA.p.sub.2.
According to the invention it is possible to position a circulation mechanism or route for solids from the circulating fluidized bed via a slow fluidized bed with extended heat transfer surface area by utilizing the different pressure conditions of the circulating fluidized bed and slow or bubbling fluidized bed. By appropriately positioning the chambers and openings through which they communicate with each other it is possible to maintain and control the operation of fluidized bed reactor so that efficient cooling of solid material is established in a safe and reliable manner under all operating loads, but specifically also at low load conditions.
According to one aspect of the present invention a fluidized bed reactor system is provided which comprises the following elements: A fluidized bed reaction chamber comprising a circulating fluidized bed, having a first grid for the introduction of fluidizing gas into the circulating fluidized bed. A bubbling fluidized bed having a second grid for the introduction of fluidizing gas thereinto. The second grid mounted at a position vertically below the first grid. A first interconnection between the circulating fluidized bed and the bubbling bed providing for the passage of solids from the circulating bed to the bubbling bed, the first interconnection located above the first grid at a first position. And, a second interconnection between the circulating fluidized bed and the bubbling bed providing for the passage of solids from the bubbling bed to the fluidized bed, the second interconnection located below the first interconnection, but at the level of, or above, the first grid. The circulating and bubbling beds, and interconnections therebetween, being positioned with respect to each other so that the pressure and density conditions within the beds establish the driving force to control the flow of particles frown the circulating bed to the bubbling bed through the first interconnection, and from the bubbling bed to the circulating bed through the second interconnection [there may be other, preferably non-mechanical, flow-controlling means].
Typically a cooling means, such as an indirect heat exchanger, is provided in the bubbling bed for cooling the solids therein. Also a partition divides the bubbling bed into first and second chambers, the first chamber in direct communication with the first interconnection, and the second chamber in direct communication with the second interconnection. The partition prevents short circuiting of particles between the first and second interconnections, so that all particles passing into the bubbling bed will be cooled.
The cooling mechanism may be located only in the first chamber, only in the second chamber, or in both the first and second chambers. The first and second chambers can be of any relative size, but it is preferred that the first chamber have a first cross-sectional area and that the second chamber have a second cross-sectional area which is less than 50% (preferably less than 25%) of the first cross-sectional area. Preferably the heat transfer means in the bubbling bed extend at least partly below the first grid.
Typically the reaction chamber has a first sidewall which is disposed at an angle greater than about 10 degrees with respect to the vertical, and a first interconnection comprises a first opening in the sidewall and the second interconnection comprises a second opening in the sidewall between the first interconnection and the first grid. A valved solid withdrawal conduit may be provided adjacent the second grid for selectively withdrawing solids from the bubbling bed. Also a portion of the second grid may be provided beneath, and horizontally overlapping, the first grid so that the second interconnection is disposed in an interruption in the first grid. The partition may extend only vertically within the bubbling bed, only at an angle of greater than about 20 degrees to the vertical, or first at angle and then essentially vertically.
According to another aspect of the present invention a method of operating a fluidized bed reactor system comprising a reaction chamber including a first fluidized bed and an accessory chamber comprising a second fluidized bed is provided. The method comprises the steps off (a) Operating the first fluidized bed as a fast, circulating, fluidized bed. (b) Operating the second fluidized bed as a slow, bubbling, fluidized bed. (c) Causing a first stream of particles to flow from a first interconnection location within the first bed into the second bed essentially solely because of pressure and density differences between the beds at the first interconnection location. And, (d) causing a second stream of particles to flow from a second interconnection location within the first bed from the second bed into the first bed essentially solely because of pressure and density differences between the beds at the second interconnection location.
There preferably is also the further step of cooling the particles in the bubbling bed between steps (c) and (d), as by directing them (e.g. with partitions) to flow past an indirect heat exchanger or a like cooling mechanism. Step (c) is typically practiced in a first vertical position within the circulating bed and step (d) is practiced in a second vertical position within the circulating bed lower than the first position, and the mean density in the bubbling bed is greater than the density in the circulating bed at the second interconnection.
Typically--as described above with respect to the apparatus--the reaction chamber includes a grid for introducing fluidized air into the circulating fluidized bed, and step (d) can be practiced to introduce solids from the bubbling bed into the circulating bed at a horizontal intermediate portion of the grid from below the grid.
According to a preferred embodiment of the present invention the driving force created by the difference between the second and first vertical pressure gradients and/or the solid density distributions may be advantageously utilized for transporting solids from the second fluidized bed to the first at the second interconnection by providing the second vertical pressure gradient to be greater than the first pressure gradient at the level of the second interconnection. Also, transferring heat from the solids indirectly into heat transfer medium, preferably steam or water, may be practiced in the second fluidized bed safely and efficiently. The second fluidized bed may be fluidized by using a gas (e.g. nitrogen) that establishes conditions that are favorable for safe long term operation, for example hazardous conditions caused by chlorine may be avoided. The driving force created by the difference between the first and second vertical pressure gradient and/or the solid density distribution may be advantageously utilized for transporting solids from the first fluidized bed to the second at the first interconnection by providing the first vertical pressure gradient to be greater than the second pressure gradient at the first interconnection. Thus the solids are flowing from first fluidized bed chamber to second fluidized bed chamber.
Advantageously, it is possible to extend the second fluidized bed of solids below the grid of the first fluidized bed and provide an extended heat transfer surface in the second fluidized bed of solids at a portion thereof below the first grid for transferring heat from the solids into a heat transfer medium, by establishing the movement or transportation of the solids by utilizing substantially only the driving force created by the different pressure conditions in the fluidized beds. In this manner it is possible to provide as much heat transfer area (and volume of the second fluidized bed) as required for adequate operation of the process while still making possible the introduction of solid material from the first fluidized bed, even at low load conditions when the fluidization is at reduced mode and only a small amount of solids are entrained by gas to move to a higher level. The present invention makes it possible to position the first interconnection at a vertical location where an adequate rate of solid flow may be realized even at low load conditions. And further, there is no need to provide any partition walls inside the first fluidized bed reactor when practicing the invention.
It is the primary object of the present invention to provide a simple yet efficient fluidized bed reactor system capable of efficient operation under a wide variety of circumstances. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.